Oils and fats are complex mixtures of water insoluble organic compounds derived from animal or vegetable sources. Such mixtures find utility in a variety of applications around the world. Oils can have a range of properties due to their unique compositions, which vary according to the sources from which they are derived. Oils are particularly well-suited to applications such as lubrication, heat transfer, and food preparation. For the food industry, oils are chosen based on their potential health benefits to the end user, their taste, and their physical properties.
The commercial utility of oils and fats is immense. Millions of tons of oils and fats are used per annum in edible products, including butter, margarine, lard, shortening, mayonnaise, salad oil, and cooking oil. The major components found in cooking or edible oils are primarily triglycerides, which are esters of glycerol linked with three molecules of fatty acids. The fatty acids contain a carboxyl group and a hydrocarbon chain. Individual fatty acids are distinguished from one another by the nature of the hydrocarbon chain, which can vary in length from about 4 to about 24 carbon atoms and can be saturated, monounsaturated (one double bond), or polyunsaturated (two or more double bonds). The most common fatty acids in edible oils and fats are those containing 18 carbons and those having one or more unsaturation (one or more double bond).
These oils can be applied in a working environment where high temperatures are maintained over prolonged periods of time with contact to air. The exposure to high temperatures in the presence of either oxygen or water can lead to degradation of the compounds in the oil via processes such as oxidation and lipolysis. These reactions lead to contaminants that result in an oil that becomes visibly darker in color, has an increase in malodorous components, and a less palatable flavor. This process of degradation of the oil results in rancidity. As the oil becomes rancid, the oil also begins to produce heavy smoke at normal cooking temperature and becomes unusable.
Lipolysis is the decomposition of the ester linkage in the fatty acid. This process is caused by water introduced to the system from frozen foods, for example, or even from fresh foods. The water reacts at the elevated temperatures with the fatty acids, breaking apart the ester linkage to form an acid and an alcohol.
Oxidation is due to the exposure of the oil to the oxygen present in air. Oxygen reacts adversely with the double bonds in the fatty acids. Oxygen is about five times more soluble in oil than in water. Oxygen will react with cooking oil to form many byproducts. These products include peroxides, aldehydes, ketones, epoxides and acids, to name a few.
Oil may also pick up particulates and products from the foodstuffs that are placed in the oil for cooking. These contaminants also lead to the degradation and discoloration of the cooking oil.
The combination of reaction byproducts and contamination from cooking leads to another problem in deep frying, which is the buildup of soap-like compounds. These soapy materials eventually lead to undesirable foaming oil. This property is often used as an indicator of the rancidity of the oil.
Another problem that is associated with these compounds is a build up of trace metals in the oil, which give rise to unpalatable flavors. Trace metals act as a catalyst for the reaction of oxygen and the oil compounds. Thus, the presence of these compounds will cause the oil to turn rancid at a much faster pace. Without treatment, these decomposition processes occur rapidly and ultimately will require the replacement of the cooking oil as often as every 2 or 3 days, thereby significantly raising the cost of operation of a commercial fryer.
Biodiesel is a fuel comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats. Biodiesel is typically produced by a reaction of a vegetable oil or animal fat with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters, which can be used as fuel, and glycerin, which is removed. Used edible oils such as vegetable oils have been considered as a source for the production of biodiesel. However, the production of biodiesel from used oil sources is limited by the presence of fatty acids. For example, fatty acids can poison the catalyst used to produce biodiesel from used oil.
There are several methods used in the trade to mitigate these problems, but many of these address only one of the above mentioned causes, thus failing to significantly extend the life of the oil. Some methods introduce additional health and/or safety concerns. Further, these methods do not significantly extend the utility of the oil.
There is a need for new methods for the treatment of cooking oil to extend its service lifetime. Further, such treatment methods are needed to facilitate use of vegetable oil as an important source of biodiesel.